1. Field of the Invention
The invention relates to cameras for recording images and diffraction patterns produced by transmission electron microscopes or scanning electron microscopes, and particularly to cameras comprising cooled charge-coupled devices.
2. Description of Prior Art
Images and diffraction patterns produced by transmission electron microscopes are typically observed in real time on a fluorescent screen. When the adjustment of the electron microscope has been fully optimized, the images are recorded on photographic film inserted directly into the vacuum of the electron microscope. In modern electron microscopes, the fluorescent screen is usually replaced by a TV camera which is used for real-time observation, but the photographic film is retained as the medium of choice for recording images.
The resolution, sensitivity, and number of pixels available on photographic film are generally satisfactory, but the dynamic range of photographic film is limited to typically less than 1:500, and the response of the film to different electron intensities is typically very non-linear. Further, the photographic film is not reusable, which makes it relatively costly, and must be chemically processed, which is slow and tedious. The resultant delay of typically several hours between the time the image is recorded and the time it is available for analysis is especially inconvenient when the images are to be processed and analyzed by a computer.
A better approach to recording electron microscope images and diffraction patterns is to convert the electron image to a light image in an electron scintillator, and use a charge-coupled device (CCD) of the type originally developed for astronomy to read the light image into a computer. Such cameras offer excellent resolution, sensitivity, linearity, up to 2048 by 2048 pixels, are reusable, and make the image available for processing and analysis within a few seconds after it was recorded. However, the CCD sensor must typically be cooled to about -30.degree. C. during operation in order to supress its intrinsic dark current to a level permitting exposures of several tens of seconds. This requires the sensor to be housed in a vacuum enclosure, in order to prevent ice build up which can potentially destroy it.
Two types of cameras using cooled, slow scan CCDs have been developed for electron microscopy. Both types house the CCD in a sealed vacuum enclosure which is isolated from the microscope vacuum. In the first type of CCD camera, the CCD enclosure has a glass window, and an image produced by the electron beam incident on a scintillator inside the electron microscope is transferred through the window onto the CCD using glass lenses. Unfortunately, the poor light collection efficiency of glass lenses prevents this type from attaining the sensitivity necessary for recording faint electron images, especially images from radiation-sensitive materials. Further, the vacuum inside the CCD enclosure typically degrades over time, and must be periodically renewed using an auxiliary vacuum station.
In the second type of CCD camera, the vacuum enclosure has a fiber-optic window. One side of the window is inside the microscope vacuum, and is coupled to a scintillator. The other side is inside the CCD vacuum, and is coupled to the CCD. This type attains a higher optical coupling efficiency and therefore a better sensitivity. However, the fiber-optic plate is in thermal contact with both the CCD and the wall of the vacuum enclosure, and this reduces the CCD cooling efficiency to the point where liquid nitrogen must be used to attain a sufficiently low operating temperature. Further, to prevent water condensation on the scintillator which is also in thermal contact with the CCD via the fiber-optic plate, the plate must be quite thick so that the scintillator can stay close to room temperature while the CCD is at a low temperature. Thick fiber-optic plates typically contain many broken fibers and other optical defects, and this significantly degrades the quality of the image recorded by the CCD. Finally, just like with the first type, the vacuum inside the enclosure degrades and must be periodically renewed.
Most users of electron microscopes would find significant advantage in an image recording device combining the good sensitivity of a fiber-optically coupled CCD with the defect-free image quality possible when thick fiber-optic plates are not used. They would also find advantage in a CCD camera whose internal vacuum did not need periodic renewal, and which attained a low enough temperature without the use of liquid nitrogen.